The technology of data migration is well known. Data migration is generally performed in units of volumes. For example, according to the Citation 1, all data stored in the first logical volume is migrated from the first logical volume to the second logical volume.
The technology of storage hierarchization is also well known. For example, according to the Citation 2, each of multiple logical volumes which multiple storage systems comprise is classified in any one tier of multiple tiers, and the logical volumes belonging to a certain tier are migrated to another tier. Multiple tiers, for example, include highly-reliable tiers, low-cost tiers and others.